1. Field of the Invention
The present invention relates to the configuration of various components in electrochemical cells for the generation of fluorine by electrolysis of a fused potassium fluoride--hydrogen fluoride electrolyte. In a further aspect, this invention relates to a process for the operation of an electrochemical fluorine cell.
2. Description of the Related Art
In the electrolytic production of fluorine gas, the reaction vessel in which the reaction occurs is commonly referred to as a "cell". The major components of a cell usually comprise the following six elements. First, an electrolyte resistant container (case) normally jacketed with a temperature control system. Second, an electrolyte, operated in the fluid state (melt), typically comprising about 39 to 42% hydrogen fluoride, although concentrations outside this range are acceptable.
Third, in some cells, a cathode is made an integral part of the cell case, while fourth, an anode is typically made of ungraphitized carbon. The carbon can have either low-permeability or high-permeability, and may be composed of a monolithic structure or a composite structure. Nickel anodes are also occasionally used. Fifth, a gas separation assembly, which captures a cathode produced gas, H.sub.2, in a chamber and an anode produced gas, F.sub.2, in a chamber above the melt and separated by a metal wall or skirt. The skirt extends from the top of the cell (cell top) below the normal operating surface of the melt. Some cells also have a sixth (6) component which is a separation diaphragm that extends from the bottom of the skirt below the melt surface to below the end of the anode. The diaphragm is made of a porous media. The diaphragm provides a means for the separation of gas bubbles of hydrogen (formed at the cathode) and the generated fluorine (formed at the anode), to prevent spontaneous and often violent reforming of hydrogen fluoride, as disclosed in U.S. Pat. No. 4,602,985 issued to Hough. The configuration of each of these components and the characteristics of the materials used therefor determines the efficiency and life of each.
In the majority of the commercially operated fluorine cells the anodes are ungraphitized carbon blades, having planar or flat surfaces, are approximately 8 inches wide by 2 inches thick and hang down vertically from 10 to 29 inches in length. These blades are normally bolted to a copper buss bar inside the cell, or are suspended individually through the cell head and fastened to a hanger assembly, as in U.S. Pat. No. 5,688,384 Hodgson. Both these methods and others connect the power supply posts (rods), by penetrating the carbon blade either by bolting the blade to the buss through a drilled horizontal hole in the carbon, or by another method of drilling and tapping a vertical hole into the carbon and the power supply rod is screwed into the hole. These carbon to metal connections are frequently the source of high cell maintenance and short cell life cycles. The large flat face of these anodes are mounted parallel to the flat surface faces of the cathode plates.
The production capacity of a fluorine gas generator cell is commonly understood to be a factor of the quality of the carbon in the anode, and of its ability to withstand a passage of a given electrical current density (as measured in amperes per square inch of interactive surface area) between the parallel interactive surfaces of the anodes and the cathodes. Operating at higher current densities can cause the anodes to degrade and burn away as CF.sub.4 gas. Therefore the total interactive surface area for each anode in a cell times the number of anodes in that cell, will determine the maximum amperage that can be applied to the cell safely. Thus, the fluorine production capacity of the cell is determined by the surface area of the anode.
In the majority of the commercially operated fluorine cells the cathode plates are mounted in a fixed position parallel to the two large anodes faces of each anode blade. The cathode is suspended from posts that penetrate the cell head through isolating packing couplings. This configuration is frequently a factor in poor cell performance. The configuration of the cell head chamber separating skirts, their position between the anodes and cathodes and the depth below the varying electrolyte melt level individually and collectively effect production capacity, product quality, and cell life cycle time.
The configuration and location of the anodes, cathode and skirts with respect to each other all effect the circulation of the electrolyte melt in the cell. The most commonly used fluorine cells today do not have a designed melt circulation path providing beneficial melt temperature control, gas bubble separation into proper chambers, and proper mixing of the hydrogen fluoride feed into the melt. All these factors result in poorer than optimal performance.
The anode hanger support is a carbon to metal connection, one of the primary keys to a long fluorine cell cycle life. In order to maximize the cycle life, there are three major problems which must be overcome.
1. The anode-support connection is subject to contamination by melt creeping into the joint. PA0 2. The fluorine cells commonly in use today have a high current density at the carbon to metal interface. This connection is normally placed under the surface of the electrolyte melt to help dissipate the heat, but this results in the melt creeping into the joint thereby degrading the electrical connection, creating hot spots, and shortening the cycle life. PA0 3. In the majority of the commercially operated fluorine cells present day, an individual cell has banks of anodes that operate in parallel to each other on each bank, but in series to anodes on the other bank. The failure of one anode can have a dynamic shift in current density to the other anodes on that bank of anodes, leading to early failure of the anodes forced to carry the extra load. The fluorine cell components are normally located inside the cell case. The cell case is normally a rectangular box shaped container with a top flange so everything nests inside of the case and supported at the case flange.
The case normally rests on support legs or wheels and has electrical isolation pads between each support to prevent current flow to ground. The case is normally used in maintaining a controlled temperature of the melt inside of the case. The cell case walls are normally jacketed with heat exchanger panels, so heating or cooling fluid media may pass through the heat exchange panels, regulating the melt temperature. In some cells, the heat exchange media is passed through tubes inside the cell case to assist in controlling the melt temperature. Heating temperature control occasionally is applied to the bottom of the case with electrical heating elements. In some cases, the cell case itself is used as the cathode for the cell.
An electrical isolation barrier (such as a sheet of plastic material like PTFE) is placed over the bottom of the cell so as to prevent cathodic interaction with the cell floor and the anode blade(s). Such a component prevents electrolytic interaction from the bottom of the cell up to the anode blades. Such an interaction risks producing both hydrogen and fluorine gases proximate one another. Such cathodic interaction would result in gases which could not be separated, potentially resulting in uncontrolled recombination of the gases, both a potentially hazardous condition and at best a waste of energy.
In prior art the cathodes are supplied power by way of posts that pass through the head plate or through the cell case. Prior art only utilizes two parallel anode surfaces for interactive current flow, not fully utilizing the all available anode surface area. However, the prior art does not supply power through a flange plate that is electrically isolated from the head plate and the case as in the instant invention.
Some cell cases are equipped with special sight glass port windows to allow visual observation of the melt levels and any other activity in the hydrogen side gas chamber of the cell, thus permitting persons to monitor the electrolyte level.
U.S. Pat. No. 5,688,384 issued on January 1997 to Hodgson discloses anodes of ungraphitized carbon blades, planar or flat surfaced, bolted to copper buss bar inside cell fastened to a lug assembly, power supply, bolted rods penetrating a drilled/boring hole in carbon or by method of drilling and tapping a vertical hole into carbon & power supply rod screwed in the hole. The holes are a source of maintenance problems. U.S. Pat. No. 5,378,324 issued on January 1995 to Hodgson focuses on macroscopic elements of a fluorine collection system. P.C.T. application WO 96/08589 published March 1996 to British Nuclear Fuels, discloses a system for electrolysis of fluorine focussing on the collection chamber. E.P.O. application EPO 852 267 A2 to British Nuclear Fuels discloses nickel coating of the joint for integrity, a nut and bolt or screw attachment of anode to a hanger.
U.S. Pat. No. 3,069,345 issued on December 1962 to Lowdermilk discloses the use of a membrane boot to seal the joint from the fluorine. Regarding a clamp supported by mechanical compressions, the patent discusses the seal/joint integrity problem of electrical contamination, parallel current source, and a shrinking of the electrode to create the seal.
U.S. Pat. No. 3,437,579 issued on March 1966 to Smith discloses horizontal electrodes. U.S. Pat. No. 3,708,416 issued on January 1973 to Ruebner discloses porous electrodes for greater electrode surface area. U.S. Pat. No. 3,752,465 issued on August 1973 to Siegman discloses a rotatable cam clamping means for moving electrodes into and out of solution, with a nut and bolt securing means. U.S. Pat. No. 4,046,664 issued on September 1977 to Fleet discloses a fibrous electrode to increase the electrode surface area. U.S. Pat. No. 3,773,644 issued on November 1973 to Tricoli discloses using a gas proof coating to maintain anode joint integrity. U.S. Pat. No. 4,139,447 issued on March 1976 to Faron discloses parallel electrodes. U.S. Pat. No. 4,176,018 issued on November 1979 to Faran discloses using electrodes in parallel. U.S. Pat. No. 4,203,819 issued on May 1980 to Cope discloses a flow detection means.
U.S. Pat. No. 4,357,226 issued on November 1982 to Alder discloses anodes with abutting aluminum connections perpendicular to the anode, and a joint above solution. U.S. Pat. No. 4,511,440 issued on April 1985 to Sparakhisn discloses expanded surface area through holes in electrode.
U.S. Pat. No. 4,950,370 issued on August 1990 to Tarancan discloses parallel anodes, a pump and flow mechanism increasing efficiency of the electrolysis through active circulation of the electrolyte, and the use of horizontal electrodes sandwiching a two side electrode between a cathode, the connection and sealing internal to brushes in the vessel. U.S. Pat. No. 5,085,752 issued on February 1992 to Iwanga discloses a methodology for collecting fluorine gas. U.S. Pat. No. 5,290,413 issued on March 1994 to Bauer discloses overcoming connection failure by coating the connection, and purging fluorine from joint, and the use of parallel electrodes. U.S. Pat. No. 5,366,606 issued on November 1994 to Taracon discloses gas collection chambers.
The published book by Rudge, A. J., "The Manufacture and Use of Fluorine and Its Compounds," pp. 18-45, 82-83, Oxford University Press (1962) which is incorporated herein by reference, discloses the use of porous anodes for enhanced surface area, and the use of MONEL.TM. skirts in the separation of the hydrogen and fluorine gas into their respective chambers, and the use of a cooling jacket to heat and cool the electrolyte melt to operating temperature, but does not teach the creation of an electrolyte melt flow circulation.
The published book by D. Van Nostrand Company, Uranium Production Technology, pp. 469-473, Colonial Press (1959), discloses the basic construction and use of fluorine generation cells.
None of the above inventions and patents, taken either singularly or in combination, is seen to describe the instant invention as claimed. Thus a fluorine gas generation system solving the aforementioned problems is desired.